1. Field of the Invention
This invention relates in general to equipment for handling drill pipe on a drilling rig, and in particular to equipment for lifting drill pipe from the rig floor for connection to a power drive carried in the derrick.
2. Description of the Prior Art
A top drive drilling rig uses a driven drive stem carried in the derrick for rotating the drill pipe. This differs from the majority of drilling rigs, which have a driven rotary table on the rig floor through which a square kelly passes for rotating the drill pipe. One advantage of a top drive system is that it enables the driller to connect a triple stand of three joints or sections of drill pipe onto the top of the drill string. He then will drill the triple stand down to a point next to the rig floor, then add another triple stand. With conventional rotary table rigs, normally only a single joint can be drilled down at one time because of the length of the kelly.
One problem, however, with top drive drilling rigs is in connecting the stand of drill pipe to the drive stem. The drive stem will be more than ninety feet above the rig floor while making the connection. Normally the driller will lower the drive stem until it contacts the upper end of the stand. Then he will rotate the drive stem to secure it to the stand. A stabbing bell is mounted around the drive stem to align the drive stem with the stand as the drive stem is lowered onto the stand. Nevertheless, cross threading is not infrequent. This damages the threads.
One apparatus employed to avoid such occurrence is mounted in the derrick below the drive stem and is described in U.S. Pat. No. 4,667,752, Joe R. Berry et al., issued May 26, 1987. This device grabs a stand of drill pipe, picks it up and inserts it into engagement with the drive stem. While successful, the apparatus adds additional weight in the derrick and requires power to be supplied to the apparatus in the derrick. Pipe handling devices for mounting to a rig floor are known in general, but not specifically for top drive drilling rigs.